International audienceWe analyse the ability of CMIP3 and CMIP5 coupled ocean-atmosphere general circulation models (CGCMs) to simulate the tropical Pacific mean state and El Niño-Southern Oscillation (ENSO). The CMIP5 multi-model ensemble displays an encouraging 30 % reduction of the pervasive cold bias in the western Pacific, but no quantum leap in ENSO performance compared to CMIP3. CMIP3 and CMIP5 can thus be considered as one large ensemble (CMIP3 + CMIP5) for multi-model ENSO analysis. The too large diversity in CMIP3 ENSO amplitude is however reduced by a factor of two in CMIP5 and the ENSO life cycle (location of surface temperature anomalies, seasonal phase locking) is modestly improved. Other fundamental ENSO characteristics such as central Pacific precipitation anomalies however remain poorly represented. The sea surface temperature (SST)-latent heat flux feedback is slightly improved in the CMIP5 ensemble but the wind-SST feedback is still underestimated by 20-50 % and the shortwave-SST feedbacks remain underestimated by a factor of two. The improvement in ENSO amplitudes might therefore result from error compensations. The ability of CMIP models to simulate the SST-shortwave feedback, a major source of erroneous ENSO in CGCMs, is further detailed. In observations, this feedback is strongly nonlinear because the real atmosphere switches from subsident (positive feedback) to convective (negative feedback) regimes under the effect of seasonal and interannual variations. Only one-third of CMIP3 + CMIP5 models reproduce this regime shift, with the other models remaining locked in one of the two regimes. The modelled shortwave feedback nonlinearity increases with ENSO amplitude and the amplitude of this feedback in the spring strongly relates with the models ability to simulate ENSO phase locking. In a final stage, a subset of metrics is proposed in order to synthesize the ability of each CMIP3 and CMIP5 models to simulate ENSO main characteristics and key atmospheric feedbacks

International audienceTrends in observed sea surface salinity (SSS) and temperature are analyzed for the tropical Pacific during 1955–2003. Since 1955, the western Pacific Warm Pool has significantly warmed and freshened, whereas SSS has been increasing in the western Coral Sea and part of the subtropical ocean. Waters warmer than 28.5°C warmed on average by 0.29°C, and freshened by 0.34 pss per 50 years. Our study also indicates a significant horizontal extension of the warm and fresh surface waters, an expansion of the warm waters volume, and a notable eastward extension of the SSS fronts located on the equator and under the South Pacific Convergence Zone. Mixed layer depth changes examined along 137°E and 165°E are complex, but suggest an increase in the equatorial barrier layer thickness. Our study also reveals consistency between observed SSS trends and a mean hydrological cycle increase inferred from Clausius–Clapeyron scaling, as predicted under global warming scenarios. Possible implications of these changes for ocean–atmosphere interactions and El Niño events are discussed

The El Niño-Southern Oscillation (ENSO) influences climate and weather over much of the globe, so uncertainties in its response to external forcing 1 hinder long-range climate predictability. Modeling studies have argued that such forcings may affect ENSO either via the seasonal cycle [2][3][4][5][6] or extratropical influences [7][8][9] , but this link is poorly constrained by the short instrumental record. Here we synthesize a pan-Pacific network of high-resolution marine biocarbonates spanning discrete snapshots of the Holocene (past ∼ 10,000 years), which we use to constrain a set of global climate model (GCM) simulations via a forward model 10 and a consistent treatment of uncertainty. Observations suggest important reductions in ENSO variability throughout the interval, most consistently during 3-5 kyBP, when ∼2/3 reductions are inferred. The magnitude and timing of these ENSO variance reductions bear little resemblance to those simulated by GCMs, or to equatorial insolation. The central Pacific witnessed a notable mid-Holocene increase in seasonality, at odds with the reductions 11 simulated by GCMs. Finally, while GCM aggregate behavior is consistent with an inverse relationship between seasonal amplitude and ENSO-band variance in sea-surface temperature 3,6,12, 13 , this relationship is not borne out by these observations. The synthesis suggests that tropical Pacific climate is highly variable, but exhibited millennia-long periods of reduced ENSO variability whose origins, whether forced or unforced, are a crucial issue for model development and long-term climate prediction.2 ENSO, the non-linear interaction between the tropical Pacific atmosphere and ocean, is the leading pattern of global interannual variability, with important physical, ecological, and human impacts. Yet, predicting its long-term behavior in the face of continued greenhouse forcing has proven elusive 1 . While the predictive skill of climate models at interannual timescales can be tested using instrumental observations, such records are too short to evaluate the fidelity of modelsimulated tropical Pacific variability on decadal-to centennial-timescales, i.e. those relevant for future climate projections. This motivates the use of paleoclimate observations, which cover a much longer time span and predate the observations used to develop and tune climate models, hence providing an out-of-sample test of their predictive ability 14 .The mid-Holocene (MH, ca 6,000 yrs before present; 6 kyBP) represents a key target for evaluating the simulated response of ENSO to changes in external forcing. While ice volume and greenhouse gas concentrations were essentially similar to today, the latitudinal and seasonal distribution of incoming solar radiation (insolation) was markedly different as a result of precession 15 : seasonal contrast was amplified in the northern hemisphere and reduced in the southern hemisphere. Thus, the mid-Holocene provides an opportunity to explore the link between changes in the seasonal cycle, meridional asymmetry in th...